Recently, as magnetic recording media have been used in increasingly various fields, electromagnetic properties and various other characteristics have been required to be improved.
For this purpose, various studies and improvements have been made to improve a binder composition in a magnetic material.
However, hitherto obtained binder compositions have both merits and demerits, and satisfactory binder compositions meeting various combination requirements have not yet been obtained.
As binders for a magnetic recording medium, cellulose derivatives, vinyl chloride/vinyl acetate copolymers, polyurethane resins, acrylic type resins and copolymers thereof, vinylidene chloride and copolymers thereof, epoxy resins, phenoxy resins, synthetic rubbers, and polyesters have been conventionally used, alone or in combination. Among these binders, the combination of a polyurethane resin and a vinyl chloride/vinyl acetate copolymer, and that of a polyurethane resin and nitrocellulose are typical examples of binders used, as described in U.S. Pat. Nos. 4,414,288, 4,423,115, 4,431,700 and 4,409,291.
However, the above combinations have been unable to fully satisfy desired properties for dispersibility of magnetic particles and wear resistance. Particularly, where fine ferromagnetic powders having a BET specific surface area of 30 m.sup.2 /g or more are used, it has not been possible to attain a satisfactory friction coefficient in addition to satisfactory dispersibility and wear resistance.
In order to solve the above problems, it has been described, for example, in Japanese Patent Application (OPI) Nos. 41435/83, 13519/81, 105429/83, 105430/83, 70424/83, 70425/83 and U.S. Pat. No. 4,423,115 that magnetic recording media using various binders have been developed. So, although the above problems have partially been solved by the above prior arts, but deterioration of the S/N (signal/noise) ratio under harsh conditions of a higher temperature (higher than 30.degree. C.) and a higher humidity (higher than 65%RH) has not been prevented nor reduced.